climbing up the string continues beyond the momentary application of the tug. As the yo-yo continues to climb back up the string, the angular momentum (rotational kinetic energy) of the yo-yo is converted back into gravitational potential corresponding to the increasing height of the center of mass of the yo-yo. For this reason, the yo-yo's rotational kinetic energy and, hence, its rotation rate, steadily decreases as the yo-yo rises. This is, of course, the reverse of the process when the yo-yo was dropped. If not for frictional losses, the yo-yo would climb all the way back up the string to your hand just as its rotational rate decreases to zero. But, due to friction, the yo-yo does not quite make it all the way back up to your hand before it stops rotating. Thereafter, the process repeats, with the yo-yo returning short of its previous height on each cycle. Eventually, the yo-yo comes to rest at the bottom. Of course, as everyone knows, it is possible to keep the yo-yo going indefinitely by giving it a slight upward pull on each cycle. This pull can be combined with the tug required to initiate the climb back up the string. The pull serves to give the center of mass of the yo-yo a little extra kinetic energy to compensate for frictional losses, so that the Wallin, 3yo-yo can be kept going indefinitely. Yo-yos can also be thrown horizontally, or launched in other directions. The principle of operation is then just the same except that the kinetic energy of the center of mass, which is converted into spin as the string unwinds, results from being thrown, rather than from falling through a gravitational potential. ...
